Power disturbances on a power grid system are not limited solely to lighting-induced voltage surges as previously thought. Voltage disturbances on power grid systems may embody other common forms such as voltage sags, momentary interruptions, overvoltages, undervoltages, and power outages. Voltage sags on the power grid may occur when short circuits occur somewhere on the grid, a large load is connected to the grid, a sudden load change, etc.
Voltage sags may account for 90-95% of power disturbance events, particularly in commercial and industrial facilities. Voltage sags are voltage reductions in the nominal line voltage that persist for a particular time period. Typically, voltage sags are characterized by drops of between 10%-90% of nominal (system) line voltages. The drops in voltage typically last from a cycle (16.6 millisecond) to a second or so, or tens of milliseconds to hundreds of milliseconds.
Certain method and apparatus for correcting certain types and forms of voltage sags are known. One example is U.S. patent application Ser. No. 13/449,870, by Deepakraj Divan, filed Apr. 18, 2012, entitled “Voltage Sag Corrector Using a Variable Duty Cycle Boost Converter,” assigned to the same assignee as the present application. This patent application and its contents are incorporated by reference herein as if made a part hereof. While voltage sag correction is one method of addressing power disturbances having certain properties, such voltage sag correctors may not be suitable for many industrial applications.
Certain methods and apparatus for reducing inrush current to electrical loads due to voltage sags are also known. For example, U.S. Pat. No. 8,039,994, of Deepakraj Divan, entitled Reduction of Inrush Current Due to Voltage Sags, owned by Georgia Tech Research Corporation and licensed to the assignee of this application, and its associated patent family, is one such example. This patent and its contents are incorporated by reference herein as if made a part hereof.
Other examples include U.S. Pat. No. 8,035,938, Active Current Surge Limiters, of Deepakraj Divan, also owned by Georgia Tech Research Corporation and licensed to the assignee hereof, and its associated patent family. This patent and its contents are also incorporated by reference herein as if made a part hereof.
Another example includes U.S. Pat. No. 8,411,403, Voltage Surge and Overvoltage Protection with Current Surge Protection, also of Deepakraj Divan, also owned by Georgia Tech Research Corporation and licensed to the assignee hereof, and its associated patent family. This patent and its contents are also incorporated by reference herein as if made a part hereof.
The need to identify the causes of different voltage disturbances and protect electrical loads have influenced the advent of intelligent power protection devices that monitor input line voltages for fluctuations and implement various forms of remediation or protection from resultant phenomena such as inrush currents. Some known power protection devices reside near a source voltage and some are located closer to a load, and attempt to monitor the voltage as it is supplied to the electrical load. Some intelligent power protection devices attempt to reduce “nuisance trips” (i.e., unwanted disconnections of the connected electrical load from the power source) by implementing delays or averaging techniques of the incoming supply voltage.
Most buildings, structures, or facilities have an input power feed from a main power distribution line that feeds a central connection point (e.g., fuse panel, breaker panel, distribution panel, etc.). A branch circuit is the power feeder circuit that leaves the central connection point and provides electrical power for at least one electrical load. As will be generally understood, more than one electrical load may be present on a single branch circuit, wherein multiple electrical loads are connected in parallel. Voltage drops or voltage sags are known to occur between the central connection point and the entry point of the load. There are multiple causes for a branch circuit-based voltage sag, such as incorrect feeder wire size, wherein the improper wire impedance being inserted into the circuit can cause a greater than necessary voltage drop. Additionally, when an electrical load starts, specifically an inductive electrical load, the voltage on the branch circuit will decrease. Further, if there is more than one electrical load on the branch, the reduced voltage is also seen by the each electrical load on the branch.
However, some of the remediation techniques do not incorporate the working state of the load device into consideration prior to remediation of the voltage disturbance. For example, with some mission-critical electrical load devices such as production printers or manufacturing equipment, it may be desirable to either inhibit remediation or relax the disturbance detection parameters based on the working state of the protected electrical load in order to allow the protected electrical load device to complete its required task. Further, allowing relaxed parameters may not disturb the load equipment if conditions momentarily fall below less than desirable conditions. As is generally understood, inserting a current limiting device before voltage recovery reduces the current inrush. Furthermore, if the electrical load is causing a branch circuit-based sag, additional current limitation is not necessary as it further starves the electrical load of needed voltage, which may cause an unintentional nuisance trip.
Grid-based voltage sags have a tendency to starve electrical loads of voltage, which generally discharges the internal capacitance of the load device. At the recovery point from a brief voltage sag, power interruption, or power outage, electrical loads pull a large current inrush in order to re-energize internal components. The current inrush is usually unabated, as the internal current limiting component commonly used by the electrical device to limit the current at turn-on does not have time to reset during these brief voltage sags or power outages. One form of power disturbance remediation is to insert an electrical impedance following a voltage sag, power disturbance, etc. The electrical impedance can be a standard resistor, a negative temperature coefficient resistor (NTC or thermistor), or any other resistive or reactive component inserted in to the circuit to reduce the amount of current inrush that occurs during the recovery of a power disturbance.
Some current limiting circuits utilize a single impedance source to compensate the onset of a current inrush to a load or circuit. This single impedance may not contain the most appropriate value to limit the amount of current drawn due to the nature of the particular power disturbance occurrence. Regardless the cause of the current inrush (recovery from voltage sag, recovery from power outage, recovery from a undervoltage, reconnect after an overvoltage, etc.), the value of the single impedance source inserted to reduce the current surge can cause the voltage presented to the electrical load to momentarily reduce due to the voltage drop across the inserted resistive element. This reduction in voltage can sometimes cause the connected electrical load to unintentionally turn off (e.g., nuisance trip) due to the voltage presented to the electrical load to drop below the required level for operation. An unintentional loss of power to critical load systems may potentially be severely damaging and costly to operations.
Therefore, the applicant believes there is a long-felt but unresolved need for a system or method that monitors various input line voltages and effectuates various remediation techniques more flexibly. Moreover, it is believed there is a need for a system or method to effectively recognize potential causes for nuisance trips and adjust detection parameters to enable efficient remediation measures. Further, it is believed there is a need for a system and method that can appropriately modulate inserted impedance values of power-disturbance remediation circuits.